1. Field of the Invention
The present invention relates to an electrically operated power steering control unit for improving the safety of control when two systems of motor torque direction determine device are provided.
2. Description of the Prior Art
A conventional electrically operated power steering control unit is proposed in Japanese Unexamined Patent Publication (Kokai) Hei-1-257676.
The conventional electrically operated power steering control unit disclosed in the above publication is shown in FIG. 5.
The conventional electrically operated power steering control unit includes a steering torque detector 1 (referred to as a torque detector hereinafter), motor control circuit 2, motor 3, and clutch 4.
The torque detector 1 is used for detecting a torque generated when the steering wheel of an automobile is operated.
In accordance with this steering torque, an assist torque is determined by the motor control circuit 2, and the motor 3 generates this assist torque to assist the steering shaft with rotation. Then the rotation of the motor 3 is transmitted to a steering shaft through the clutch 4 and a reduction mechanism (not shown in the drawing). In this way, the assistant steering power is supplied to the steering shaft in the steering operation.
Next, the construction of the motor control circuit 2 will be explained with reference to the operation of each component.
The output of the torque detector 1 is sent to input terminals 5A, 5B, and inputted into a microcomputer 7 (referred to as a CPU hereinafter) as a control means directly or through a phase compensation circuit 6. The phase compensation circuit 6 conducts a phase-compensating operation on an output signal corresponding to the steering torque sent from the torque detector 1, and then the compensated signal is sent to the CPU 7.
A signal outputted from the CPU 7 is sent to a rightward interface circuit 8 for driving the motor to the right, a leftward interface circuit 9 for driving the motor to the left, and a digital-to-analog conversion circuit 10 in accordance with the direction and magnitude of the steering torque. A second rightward drive signal 8B for driving the motor to the right sent from the CPU 7 is inputted into a motor drive circuit 11 through the rightward interface circuit 8. In this connection, a first rightward drive signal for driving the motor to the right will be described later. In the same manner, a second leftward drive signal 9B for driving the motor to the left sent from the CPU 7 is inputted into the motor drive circuit 11 through the leftward interface circuit 9. In this connection, a first leftward drive signal for driving the motor to the left will be described later.
The output of the motor drive circuit 11 is sent to the motor 3 through output terminals 12A and 12B.
Although not illustrated in the drawing, the motor drive circuit 11 is constructed in the following manner: For example, four power elements such as four pieces of FET are connected to form a bridge circuit, and two of the power elements are used for driving the motor to the right, and the residual two of the power elements are used for driving the motor to the left. The second rightward drive signal 8B is inputted into one of the two power elements for driving the motor to the right, and the first rightward drive signal 8A, which will be described later, is inputted into the other power element. In the same manner, the first and second drive leftward signals 9A and 9B are inputted into the two power elements for driving the motor to the left. Only when two pieces of FET used for the rotation in the same direction are concurrently inputted with a signal, the motor generates a torque so as to be rotated in that direction.
In this connection, reference numeral 13 is an error amplifier, and one of the input terminals of the error amplifier 13 is inputted with a torque output signal of the CPU 7 after the torque output signal has been converted into an analog signal by the digital-to-analog conversion circuit 10. The other input terminal of this error amplifier 13 is inputted with an output signal outputted from a motor current detection circuit 14 by which a current corresponding to an actual assist torque is detected when a voltage corresponding to the assist torque sent from the motor drive circuit 11 by the action of a resistor 15 for detection, is taken in. Then the error amplifier 13 amplifies an error between the output of the digital-to-analog conversion circuit 10 and the output of the motor current detection circuit 14, and the amplified error is outputted into one of the input terminals of a PWM (pulse width modulation) modulator 16 of the next stage.
The other input terminal of the PWM modulator 16 is inputted with an output of a PWM oscillation circuit 17 which is used as a reference oscillator. In the PWM modulator 16, the output of the error amplifier 13 and the output of the PWM oscillation circuit 17 are compared, and a PWM signal of the motor 3 is generated for the motor drive circuit 11.
Reference numeral 18 is a clutch driver, which receives an output from the CPU 7 and usually sends an ON signal to the clutch 4 through output terminals 18A, 18B so that the motor 3 and the steering shaft can be connected.
Reference numeral 19 is a motor rotational direction determining device, which includes a torque signal direction determining circuit 19A. This torque signal direction determining circuit 19A is inputted with an output signal of the torque detector 1. By this output signal, the rightward and leftward drive signals 19B and 19C are outputted from the torque signal direction determining circuit 19A. This rightward drive signal 19B is inputted into the second input terminal of an AND circuit 20A of two inputs which is a coincidence determining device, and the leftward drive signal 19C is inputted into the second input terminal of an AND circuit 20B.
Also, the first rightward drive signal 8A outputted from the rightward interface circuit 8 is inputted into the first input terminal of the AND circuit 20A, and the second leftward drive signal 9A outputted from the leftward interface circuit 9 is inputted into the first input terminal of the AND circuit 20B.
When the two inputs are in an AND condition, the outputs of the AND circuits 20A and 20B are used for directing the drive direction of the motor 3, and these outputs are sent to the motor drive circuit 11.
Next, with reference to FIG. 6, a specific example of the torque signal direction determining circuit 19A will be explained as follows.
In the drawing, reference numeral 21 is an input terminal into which an output signal of the torque detector 1 shown in FIG. 5 or a signal subjected to phase compensation by the phase-compensation circuit 2 is inputted. This input terminal 21 is connected with a positive (+) input terminal of a comparator 22 which is a rightward signal detection circuit, and also connected with a negative (-) input terminal of a comparator 23 which is a leftward signal detection circuit. Resistors 26 to 28 for generating a reference voltage are connected in series between the positive voltage side 24 and the ground side 25. Reference voltage E1 obtained at a junction between the resistors 26 and 27 is impressed upon the negative (-) input terminal of the comparator 22. Reference voltage E2 obtained at a junction between the resistors 27 and 28 is impressed upon the positive (+) input terminal of the comparator 23. Then the rightward drive signal 19B is outputted from the comparator 22, and the leftward drive signal 19C is outputted from the comparator 23.
By the power steering control unit described above, the output characteristics shown in FIG. 7 can be obtained in the torque detector 1. In FIG. 7, the horizontal axis represents lateral steering torque, and the vertical axis represents a torque output signal. In FIG. 7, T1 is a starting point of a leftward steering torque control, and T2 is a starting point of a rightward steering torque control. A region between T1 and T2 is a dead zone (neutral region) of steering torque, the center of which is located at T0. Corresponding to the steering torques shown on the horizontal axis, the outputs shown in the vertical axis are obtained as illustrated in FIG. 7. In this case, V1 is an output of the leftward control starting point, V2 is an output of the rightward control starting point, and a point in the proximity of V0, that is, a point between V1 and V2, is an output of the neutral point. The torque output characteristics are approximately linear in the control range.
FIG. 8 shows an example of the control output characteristics of the motor. In the drawing, the horizontal axis represents a torque output signal, the vertical axis represents a motor output, and a region between torque outputs V1 and V2 is a dead zone in which the motor output is not outputted. When the leftward torque output is V1, a signal to control the motor 3 to the left is outputted. When the motor output exceeds V4, the motor output is saturated so that the motor output is controlled to be a constant value (P.sub.max). On the other hand, when the rightward torque output is V2, a signal to control the motor 3 to the right is outputted. When the motor output exceeds V3, the motor output is saturated so that the motor output is controlled to be a constant value (P.sub.max). The motor output is approximately linearly changed between V1 and V4, and also between V2 and V3. In this example, even when the torque signal is the same, a different value of the motor output is outputted according to the vehicle speed. That is, the lower the vehicle speed is, the higher the motor output is increased for controlling.
Next, the overall operations of this power steering control unit will be explained as follows.
When a key switch not shown is turned on, the electromagnetic clutch 4 is turned on, so that the output shaft of the motor 3 and the steering shaft are connected. Next, a torque signal proportional to the steering torque is outputted from the torque detector 1. Then the outputted torque signal is inputted into the CPU 7 directly or through the phase-compensation circuit 6. The CPU 7 outputs the torque signal in the form of a digital signal. Then the outputted digital signal is converted into an analog signal by the digital-to-analog conversion circuit 10.
A level of the torque signal output is judged by the CPU 7. Then the rightward drive signal is inputted into the rightward interface circuit 8 for driving the motor to the right, and the leftward drive, signal is inputted into the leftward interface circuit 9 for driving the motor to the left. The motor drive circuit 11 is inputted with rightward and leftward rotation drive signals so that the rotational direction of the motor is directed, and the torque signal is converted into an analog signal by the digital-to-analog conversion circuit 10. Then the analog signal is inputted into the error amplifier 13, and the output of the error amplifier 13 is further inputted into the PWM modulator 15 of the next stage, so that the signal is modulated by the PWM oscillating circuit 16, and then a control signal, the pulse width of which is proportional to the output of the torque detector 1, is sent to the motor drive circuit 11. When necessary, the level of a motor current is restricted or cut off by the motor current detection circuit 14.
In accordance with the control characteristics shown in FIG. 8, an output signal is sent to the motor 3 from the motor drive circuit 11. According to the inputted signal, the motor 3 is rotated so that an assist power is supplied to the steering shaft.
On the other hand, the output of the torque detector 1 is inputted into not only the CPU 7 but also the torque signal direction determining circuit 19A of the motor rotational direction determining device 19. In accordance with this steering torque signal, the rightward drive signal 19B and the leftward drive signal 19C are outputted. The operation of this torque signal direction determining circuit 19A will be explained as follows:
An output signal sent from the torque detector 1 is inputted into the input terminal 21 shown in FIG. 6, and at the same time, the output signal is also inputted into the comparators 22 and 23 for the rotation of right and left. The level of the inputted signal is compared with the reference voltages E1 and E2 which are different values. When the level of the signal is higher than the reference voltage El, the signal is inputted into the AND circuit 20A as the rightward drive signal 19B, and when the level of the signal is lower than the reference voltage E2, the signal is inputted into the AND circuit 20B as the leftward drive signal 19C. In this case, the equation E1-E2=E0 is satisfied, and a voltage in the range of E0 is a dead zone voltage at which no drive signals are outputted.
The output from the torque signal direction judgment circuit 19A is inputted into the AND circuits 20A, 20B and then compared with the first rightward and leftward drive signals 8A and 9A sent from the rightward and leftward interface circuits 8 and 9.
As a result of the foregoing, only when both the first rightward drive signal 8A and the rightward drive signal 19B are inputted into the AND circuit 20A, a rightward signal is inputted into the AND circuit 20A from the motor drive circuit 11. In the same manner, only when both the first leftward drive signal 9A and the leftward drive signal 19C are inputted into the AND circuit 20B, a leftward signal is outputted into the AND circuit 20B from the motor drive circuit 11.
Due to the foregoing, for example, in the case where the CPU 7 is in trouble being affected by noise, even when an erroneous rightward signal is generated through the rightward interface circuit 8 although a rightward direction signal is not detected by the torque detector 1, a rightward direction torque is not detected by the torque signal direction determining device 19. As a result, a signal is not outputted from the AND circuit 20A. Therefore, it is possible to prevent the motor 3 from mistakenly rotating to the right. With respect to the left, the circumstances are the same.
In the conventional power steering control unit described above, while an automobile is running under the condition that the steering wheel torque is neutral in a dead zone, no drive PG,12 signals are outputted from the torque signal direction determining device 19. Accordingly, even when the CPU 7 is out of order and an erroneous signal is outputted from the CPU 7, the motor 3 is not permitted to rotate. Therefore, it is possible to prevent the steering wheel from rotating by itself. As described above, the conventional power steering control unit is excellent, however, it has the following problems.
In the conventional power steering control unit, the motor 3 is not driven in the dead zone of the torque detector 1. Therefore, for example, when the steering wheel is suddenly reversed by itself or the steering wheel is reversed without being rotated by a driver, the steering wheel temporarily rotates in the dead zone of the torque detector 1. Consequently, in this dead zone, the motor 3 is not given an assist torque. For this reason, it is not possible to improve the feeling of power steering by driving the motor in the above conditions.